1. Field of the Invention
The present invention relates to an optical waveguide, in particular, a laminated polymer optical waveguide, and a process for producing the same.
2. Description of the Related Art
As a method for producing a polymer waveguide, the following methods have been proposed: (1) a method in which a film is impregnated with a monomer, the core part is selectively exposed to light to change the refractive index and the film is then applied (selective polymerization method); (2) a method in which a core layer and a clad layer are applied and then a clad part is formed by using reactive ion etching (RIE method); (3) a method using a photolithographic method in which a ultraviolet ray-curable resin obtained by adding a photosensitive material in a polymer material is used to carry out exposure and developing (direct exposure method); (4) a method using injection molding and (5) a method in which a core layer and a clad layer are applied and then a core part is exposed to change the refractive index of the core part (photo-bleaching method).
However, the selective polymerization method (1) has a problem concerning the adhesion of the film, and the methods (2) and (3) are poor in cost performance as they employ photolithography. The method (4) has a problem concerning the accuracy of a core diameter. Also, the method (5) has the problem that a sufficient difference in refractive index between the core layer and the clad layer cannot be obtained.
At present, only the methods (2) and (3) are practical methods having a high performance in terms of quality. However, these methods have a cost problem as mentioned above. Also, any of the methods (1) to (5) is not applicable to the formation of a polymer optical waveguide on a flexible plastic substrate having a large area.
Also, as a method for producing a polymer optical waveguide, a method is known in which a polymer precursor material for core is filled in a patterned substrate (clad) having a groove pattern formed thereon to be a capillary, then cured to form a core layer and a plane substrate (clad) is applied to the surface of the core layer. However, in this method, there is the problem that the polymer precursor material is not only filled in the capillary groove but also filled thinly in the entire space between the patterned substrate and the plane substrate, the polymer precursor material applied between both substrates is cured to form a thin layer having the same composition as the core layer, resulting in that light leaks through this thin layer.
As one of methods solving this problem, David Heard has proposed a method in which a patterned substrate having a groove pattern formed thereon to be a capillary is fixed to a plane substrate by using a clamping jig, the contact part between the patterned substrate and the plane substrate is further sealed with a resin, followed by dropping the pressure of the system, and then a monomer (diaryl isophthalate) solution is filled in the capillary to produce a polymer optical waveguide (see Japanese Patent No. 3151364). This method is a method that prevents a monomer from being filled in any area other than the capillary by using the monomer in place of a polymer precursor material as a core-forming resin material to lower the viscosity of a filler material and by filling the monomer by utilizing a capillary phenomenon.
However, since this method uses a monomer as the core-forming material, it has the problem that the volume shrinkage factor when the monomer is polymerized into a polymer is large and the transmission loss of the polymer optical waveguide is increased.
Also, this method is such a complicated method that the patterned substrate is fixed with the plane substrate by a clamp and in addition to this process, the contact part is sealed with a resin, and is therefore unfit for mass-production, with the result that no reduction in cost can be expected. Also, it is impossible to apply this method to the production of a polymer optical waveguide using a film having a thickness of the order of millimeters or 1 mm or less as a clad.
George M. Whitesides et al., in Harvard University have recently proposed a method called capillary micro-mold as one of soft lithographic methods in new technologies making a nanostructure. This is a method in which a master substrate is made using photolithography, the nanostructure of the master substrate is exactly copied on a mold of polydimethylsiloxane (PDMS) by utilizing adhesiveness and releasability of the PDMS, and a liquid polymer is flowed into the mold by utilizing a capillary phenomenon and solidified. A detailed explanatory report is made on SCIENTIFIC AMERICAN SEPTEMBER 2001 (Nikkei Science, the December issue (2001)).
Kim Enoch et al., in the group of George M. Whitesides etc., in Harvard University have filed an application for a patent concerning a capillary micro-mold method (see U.S. Pat. No. 6,355,198). However, even if the production method described in this patent is applied to the production of a polymer optical waveguide, a lot of time is required to form a core part because the core part of the optical waveguide has a small sectional area, showing that this method is not suitable to mass-production. This method also has the drawback that a change in volume is caused when the monomer solution is polymerized to form a polymer, leading to a change in the shape of the core with a large transmission loss.
B. Michel et al. in IBM Zürich Research Center have proposed lithographic technologies having high resolution and using PDMS and reported that a resolution of several tens nanometers is obtained. A detailed explanatory report is made on IBM J. RES. & DEV. Vol. 45 No. 5 SEPTEMBER 2001.
As aforementioned, soft lithographic technologies using PDMS and a capillary micro-mold method are technologies on which many countries including the USA focused as nanotechnology.
However, if an optical waveguide is produced using such a micro-mold method as aforementioned, it is not impossible to make volume shrinkage factor small during curing (hence making transmission loss small) and to drop the viscosity of a filler liquid (monomer or the like) to make it easy to fill at the same time. Therefore, the viscosity of the filler liquid cannot be dropped to a certain limit or less, leading to a small filling speed and therefore mass-production is not expected taking it account preferentially to make transmission loss small. Also, the aforementioned micro-mold method is on the premise that a glass or silicon substrate is used as the substrate and it is not considered to use a flexible film substrate in this method.
Thus, the present inventors proposed, in U.S. patent application Publication No. 2004/0022499, a process for producing a laminated polymer optical waveguide comprising the steps of forming an alignment mark and an optical waveguide core section on/in each of optical waveguide films at the same time, and then using the marks to laminate the films. Objects thereof are to provide a process for a polymer optical waveguide to which an alignment mark is attached in order to make the laminating for forming the laminated optical waveguide easy, and to provide a process for producing a laminated polymer optical waveguide wherein laminating is performed using alignment marks.
U.S. patent application Publication No. 2004/0022499 discloses that an alicyclic acrylic resin film, an alicyclic olefin resin film or the like can be used as a member which constitutes a clad of an optical waveguide.
It is difficult to form optical waveguide cores in a film-penetrating direction, and thus it has been difficult to lead optical waveguide cores belonging to different laminated layers to, for example, an optical connector whose cores are arranged along a straight line at regular intervals, such as an MT connector.
As disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-183747, optical waveguides belonging to different laminated layers can be led to a single MT connector by a process of: forming optical waveguide films which each have upper and lower clad structures, the thicknesses of which are precisely controlled relative to a single-layer core section, this core section being positioned in a thickness-direction-center of the whole; and laminating the optical waveguide films, thereby arranging the sections of the optical waveguide cores along a straight line in a film-laminating direction. However, the flexibility of the arrangement of the optical waveguide cores is limited. For example, it is impossible to select only some optical waveguide cores from among the optical waveguide cores in an end face of the laminated layers, and arrange the sections of the selected optical waveguide cores along a straight line at regular intervals. This means that the amount of flexibility of arrangement is smaller than an amount of flexibility of arrangement of optical fibers in an optical fiber board produced by laying optical fibers on a polyimide film.